In one type of offshore oil or gas well, a riser is connected between a subsea wellhead and a floating drilling and/or production platform. Oftentimes, the subsea wellhead is not in exact alignment with the platform that is used to secure the riser or handle the riser during assembly. Further, the platform may be subject to movement from wave action and the like, while the riser is stationarily held at the wellhead.
It is important that relative movement between the platform and the riser, due to wave action and the like, be constrained to facilitate production and drilling operations and maintain clearance between the riser and other platform equipment. Throughout drilling and production operations, various pieces of equipment must be attached and detached from the riser, and often the riser has work platforms joined thereto which extend outward from the riser. This leaves little clearance between the riser and its surroundings, thus movement of the platform relative to the riser may cause damage to the riser, the equipment and work platforms joined to the riser, and the surrounding platform and platform equipment. Further, relative riser movement complicates the alignment and coupling of equipment which must be attached and detached from the riser.
Centralizing devices of the prior art have maintained only the riser's lateral position relative to the platform. However, it is further desirable to also constrain torsional movement relative to the platform, because it allows more freedom in the design of attached equipment or work platforms. For example, a work platform joined to a riser may be rectangular and fit in a rectangular clearance bounded by platform equipment. Rotation of the platform relative to the riser may cause the work platform to contact and damage its surroundings.